Ring-opening polymerization from cellulose for biocomposite applications

by Lönnberg, Hanna

There is an emerging interest in the development of sustainable materials with high performance. Cellulose is promising in this regard as it is a renewable resource with high specific properties, which can be utilized as strong reinforcements in novel biocomposites. However, to fully exploit the potential of cellulose, its inherent hydrophilic character has to be modified in order to improve the compatibility and interfacial adhesion with the more hydrophobic polymer matrices commonly used in composites. In this study, the grafting of poly(?-caprolactone) (PCL) and poly(L-lactide) (PLLA) from cellulose surfaces, via ring-opening polymerization (ROP) of ?- caprolactone and L-lactide, was investigated. Both macroscopic and nano-sized cellulose were explored, such as filter paper, microfibrillated cellulose (MFC), MFC-films, and regenerated cellulose spheres. It was found that the hydrophobicity of the cellulose surfaces increased with longer graft lengths, and that polymer grafting rendered a smoother surface morphology. To improve the grafting efficiency in the ROP from filter paper, both covalent (bis(methylol)propionic acid, bis-MPA) and physical pretreatment (xyloglucanbisMPA) were explored. The highest grafting efficiency was obtained with ROP from the bis-MPA modified filter papers, which significantly increased amount of polymer on the surface, i.e. the thickness of the grafted polymer layer. MFC was grafted with PCL to different molecular weights. The dispersability in non-polar solvent was obviously improved for the PCL grafted MFC, in comparison to neat MFC, and the stability of the MFC suspensions was better maintained with longer grafts. PCL based biocomposites were prepared from neat MFC and PCL grafted MFC with different graft lengths. The polymer grafting improved the mechanical properties of the composites, and the best reinforcing effect was obtained when PCL grafted MFC with the longest grafts were used as reinforcement. A bilayer laminate consisting of PCL and MFC-films grafted with different PCL graft lengths displayed a gradual increase in the interfacial adhesion with increasing graft length. The effect of grafting on the adhesion was also investigated via colloidal probe atomic force microscopy at different temperatures and time in contact. A significant improvement in the adhesion was observed after polymer grafting.